The present invention relates to a stimulable phosphor sheet employable in a radiation image recording and reproducing method utilizing stimulated emission from a stimulable phosphor.
As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screens, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. This method employs a radiation image storage panel comprising a stimulable phosphor layer (i.e., stimulable phosphor sheet) provided on a support, and the procedure of the method comprises the steps of causing the stimulable phosphor in the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with stimulating rays to release the radiation energy stored in the phosphor sheet as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The stimulable phosphore sheet thus processed is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
Generally, a substrate film and a protective cover film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor with no binder. The phosphor sheet containing no binder can be formed by deposition process or sintering process. Further, the sheet comprising agglomerated phosphor soaked with a polymer is also known. In the aforementioned method, any types of the stimulable phosphor sheets are employable.
The radiation image recorded in the stimulable phosphor sheet is generally read by the steps of applying stimulating rays onto the front surface side (phosphor layer side) of the phosphor sheet, collecting light emitted by the phosphor particles by means of a light-collecting means from the same side, and photoelectrically converting the light into image signals. A system for reading the image from one side of the panel in this manner is referred to as xe2x80x9csingle-side reading systemxe2x80x9d. However, there is a case that the light emitted by the phosphor particles should be collected from both sides (i.e., front and the back surface sides) of the phosphor sheet. For instance, there is a case that the emitted light is desired to be collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth of the layer, and it is desired to detect the variation. A system for reading the image from both sides of the phosphor sheet is referred to as double-side reading systemxe2x80x9d.
The radiation image recording and reproducing method is often used in radiography for medical diagnosis. In that case, it is especially desired to reproduce a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Thus, the stimulable phosphor sheet is required to have a high sensitivity and to provide an image of high quality.
The sharpness of radiation image is mainly affected by diffusion or scattering of the stimulating rays in the stimulable phosphor sheet. If the stimulating rays diffuse or scatter (horizontally in particular) in the phosphor sheet, it excites the phosphor not only at the target spot but also in its periphery. Consequently, the stimulated emission emitted from the target position is collected together with that from the periphery. Such noise of the emissions impairs the sharpness of the resulting radiation image.
For avoiding the diffusion or scattering of the stimulating rays, it has been proposed to divide the plane of the stimulable phosphor sheet into small sections with partitions reflecting the stimulating ray.
Japanese Patent Provisional Publication No. 59-202100 discloses a stimulable phosphor sheet of a honey-comb structure consisting of many small cells filled with a stimulable phosphor. The phosphor sheet is composed of a substrate and a stimulable phosphor layer, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 62-36599 discloses a stimulable phosphor sheet employing a support provided with many hollows regularly arranged on one surface The hollows are filled with stimulable phosphor, and the ratio of depth to diameter of each hollow is 3.5 or more.
Japanese Patent Provisional Publication No. 5-512636 discloses a process for preparing pixel phosphors with a mold.
Japanese Patent Provisional Publication No. 2-129600 discloses a storage panel employing a support plate having many holes vertically bored and filled with a stimulable phosphor.
Japanese Patent Provisional Publication No. 2-280100 discloses a stimulable phosphor sheet employing a substrate having a honey-comb micro-structure filled with a stimulable phosphor.
In each of the aforementioned known stimulable phosphor sheets employing a support or substrate provided with many holes or hollows incorporated with a stimulable phosphor, a part of the support or substrate serves as a partition preventing the simulating rays from diffusion. The stimulable phosphor sheet of this type, therefore, is useful for improving quality (particularly, sharpness) of the resulting radiation image. On the other hand, since the partition of support material partly occupies the phosphor layer, the amount of the phosphor incorporated in a unit volume of the layer is often too small to absorb enough amount of radiation. Consequently, the partition lowers the sensitivity of the stimulable phosphor sheet. Although the sensitivity can be enhanced by thickening the phosphor layer, a thick phosphor layer generally impairs the sharpness.
In U.S. Pat. No. 6,255,660, a stimulable phosphor sheet which comprising a stimulable phosphor-containing partition which divides the stimulable phosphor sheet along its plane into small sections, and a stimulable phosphor-incorporated area which is divided with the partition and which has a reflectivity with respect to the stimulating rays differing from a reflectivity with respect to the stimulating rays of the stimulable phosphor-containing partition is described.
Japanese Patent Provisional Publication No. 2-58000 describes a stimulable phosphor sheet having narrow columnar stimulable phosphor crystals which are independently and slantingly deposited on a support by a gaseous deposition method.
In radiography for medical diagnosis, the use of a stimulable phosphor sheet of high sensitivity can reduce a dose of radiation to be applied to a patient. Therefore, it is required to provide a stimulable phosphor sheet giving an image of high sharpness with high sensitivity.
Accordingly, it is an object of the invention to provide a stimulable phosphor sheet in which the diffusion or scattering of the stimulating rays is effectively avoided so as to give a radiation image having high sharpness without deterioration of graininess.
It is another object of the invention to provide industrially advantageous processes for preparing the stimulable phosphor sheet of the invention.
The present invention resides in a stimulable phosphor sheet comprising a stimulable phosphor film in which acicular particles of stimulable phosphor having an aspect ratio (in terms of a mean aspect ratio) of 1.5 or more, preferably 2.0 to 10.0, are oriented in the direction of depth of the phosphor film.
The stimulable phosphor film of the stimulable phosphor sheet of the invention preferably has a structure in which the acicular phosphor particles are enclosed with partitions containing light-reflecting material and stimulable phosphor particles.
The invention also resides in a radiation image recording and reproducing method comprising the steps of recording a radiation image as a latent image on a stimulable phosphor sheet of the invention, irradiating the phosphor sheet with stimulating rays to release stimulated collected emission from the latent image, collecting the stimulated emission, and electrically processing the collected emission to reproduce the radiation image.
The invention further resides in a process for preparing a stimulable phosphor film in which acicular particles of stimulable phosphor having an aspect ratio of 1.5 or more are oriented in the direction of depth of the film, comprising the steps of:
continuously causing a dispersion comprising the acicular particles of stimulable phosphor and a liquid medium to flow in one direction to produce a continuous film comprising a liquid medium in which the acicular phosphor particles are aligned in the direction;
drying the film of liquid film to produce a dry continuous film in which the acicular phosphor particles are aligned in the direction;
sectioning the dry continuous film to give a multiple of phosphor films;
laminating the phosphor films one on another under the condition that all phosphor films have the same alignment of the acicular phosphor particles to produce a phosphor film block in which the acicular phosphor particles are aligned in the same direction; and
slicing the phosphor film block in the direction perpendicular to the direction of alignment of the acicular phosphor particles.
The invention furthermore resides in a process for preparing a stimulable phosphor film having a structure in which the acicular particles of stimulable phosphor having an aspect ratio of 1.5 or more are oriented in the direction of depth of the film and enclosed with partitions containing light-reflecting material and stimulable phosphor particles, comprising the steps of:
continuously causing a dispersion comprising the acicular particles of stimulable phosphor and a liquid medium to flow in one direction to produce a continuous film comprising a liquid medium in which the acicular phosphor particles are aligned in the direction;
drying the film of liquid film to produce a dry continuous film in which the acicular phosphor particles are aligned in the direction;
sectioning the dry continuous film to give a multiple of phosphor films;
preparing partition films containing light-reflecting material and stimulable phosphor particles;
laminating the phosphor films and the partition films one on another under the condition that all phosphor films have the same alignment of the acicular phosphor particles to produce a block in which the phosphor films and the partition films are alternatively laminated; and
slicing the block in the direction perpendicular to the direction of alignment of the acicular phosphor particles.